Modern Radio frequency (RF) receivers used in mobile communications typically utilize either direct conversion or low intermediate frequency (low-IF) reception to receive and demodulate wirelessly received radio signals. These wirelessly received radio signals typically contain a wanted or desired signal that has been modulated onto a predefined carrier frequency. Direct conversion receivers demodulate radio signals by converting a wanted signal directly to baseband frequencies. This may be performed using a local oscillator component in an RF receiver with an oscillation frequency that substantially matches to the carrier frequency.
In contrast, low-IF receivers (also known as superheterodyne receivers) perform an initial conversion to an intermediate frequency. For example, a low-IF receiver may first down-convert a received radio frequency signal to a non-baseband intermediate frequency before performing final demodulation. Low-IF receivers may use a local oscillator with a frequency that is not identical to the carrier frequency, which results in a shift of a wanted signal from a carrier frequency to a non-baseband intermediate frequency.
Low-IF receivers offer greater simplicity due to complications involved with the high frequency circuitry required by direct conversion receivers. However, the performance of all low-IF receivers suffers from the presence of “image frequencies” which are unavoidably introduced into the intermediate frequency signal. Due to the symmetrical nature of the frequency domain, an additional unwanted frequency is also transposed to the intermediate frequency along with the wanted signal. The presence of this unwanted frequency laid overtop of the wanted signal may degrade the integrity of the wanted signal. Accordingly, low-IF receivers are usually further process the IF signal in order to compensate for the negative impacts of image frequency.